It is increasingly recognised that integrated optical circuits have a number of advantages over electrical circuits. However, it has been difficult to produce integrated optical circuits which are comparably small, primarily due to the difficulty in producing waveguides which can include tight bends without large signal losses. It has also been difficult to produce integrated optical circuits including signal processing devices based on photonic band structures which can be easily coupled to current optical fibres, owing to a difference in the refractive index of the material used for optical fibres and those materials typically used for integrated optical devices, whilst still maintaining compact sizes.
Photonic crystals comprising a lattice of air holes formed in a core material, typically silicon or silicon nitride, have been fabricated, which exhibit a photonic band structure and typically a bandgap. Alternatively, a lattice of dielectric rods in air can be used. By not including some holes or rods in the lattice a line defect waveguide can be formed. Confinement of light within the waveguide is provided by using light within the photonic bandgap wavelength range. However, it has been found that devices of this type suffer from large losses, mainly due to the escape of light from the waveguide in a vertical direction.
Similarly, optical devices using this type structure for signal processing, such as filtering, suffer from large losses. This limits their usefulness.